Active antennas have an RF circuit of a radio signal formed integrally with each element of an antenna array, and have a feature capable of controlling the radiation direction and beam shape of electromagnetic waves to be output. Particularly, in a case where the active antennas are used in a base station of moving object communication, there is an advantage in that a coverage area can be freely controlled.
As a measurement of the characteristics of an antenna having strong directivity such as an antenna which is used as such an active antenna, a near field measurement (NFM) for calculating far field directivity from the near electromagnetic field of an antenna on the basis of an electromagnetic field theory has been known.
In the near field measurement, since an electromagnetic field is measured in the vicinity of an antenna, there is an advantage in that a loss of electromagnetic waves due to a space is small, and that not only directivity can be measured but also an antenna can be diagnosed from the near field distribution of the antenna.
Generally, as shown in FIG. 12, among regions of an electromagnetic field which is radiated from the aperture plane of an antenna, a region adjacent to an antenna aperture is a reactive near field region (extreme near field) mainly containing electromagnetic field components that do not contribute to radiation, and a region having no change in directivity depending on a distance from the antenna aperture is called a radiation far field region (far field). The directivity of an antenna to be generally represented refers to directivity measured in this radiation far field region.
The far field is specified as a position away by more than a distance R which satisfies the following Expression (1) with respect to a maximum diameter D (aperture dimension) of an antenna. Here, λ is a free space wavelength. In addition, when a gain of a transmitting antenna is set to Gt, a gain of a receiving antenna is set to Gr, and transmission power is set to be Wt, maximum power Wa capable of being received by a receiving antenna in a free space is represented like the following Expression (2).R>2D2/λ  (1)Wa=(λ/4πR)2·Gt·Gr·Wt  (2)
Therefore, in an antenna having a large aperture plane with a high gain, the distance R increases, and attenuation in a space increases. Further, in a millimeter-wave zone, since a free space wavelength λ decreases in size, there is a problem in that the amount of attenuation further increases, and that it is not likely to measure a low-level side lobe.
A radiation near field region (near field) which is a region located between the reactive near field region and the radiation far field region is a region having a change in directivity in accordance with a distance. In the NFM described above, an electromagnetic field is measured in this radiation near field region, and directivity in a far field is obtained by calculation.
Specifically, the vicinity of an antenna to which a predetermined signal is supplied is scanned by a probe antenna, and the distribution of amplitudes and phases for each scanning position is obtained from a signal received by the probe antenna, thereby allowing directivity at infinity to be obtained by data processing from this distribution. Since a measurement in the vicinity of an antenna is performed, the amount of attenuation in a space is small, and thus it is possible to perform a high-accuracy measurement as compared to a far field measurement.
The NFM is divided into a plurality of types depending on a range in which the vicinity of an antenna to be measured is scanned, but is advantageous to an antenna having a high gain, and a plane NFM which is easy of data processing is widely used.
FIG. 13 shows a configuration of a measurement device that obtains the directivity of an antenna 100 to be measured using the plane NFM. The measurement device 10 includes an antenna support 51 that supports the antenna 100 to be measured in a state where its radiation plane is directed in a predetermined direction, a probe antenna 52 for receiving electromagnetic waves which are output from the antenna 100 to be measured, and a probe scanning mechanism 53 that moves the probe antenna 52 in X and Y directions within a neighboring measurement plane facing the radiation plane of the antenna 100 to be measured.
In addition, the measurement device 10 includes a signal generator 54, an amplitude and phase detector 55, a measurement control unit 56, and a display unit 57. The signal generator 54 assigns a measuring signal to the antenna 100 to be measured. The amplitude and phase detector 55 detects information of an amplitude and a phase from a received signal of the probe antenna 52. The measurement control unit 56 controls the probe scanning mechanism 53 so as to receive an output of the amplitude and phase detector 55 while scanning the position of the probe antenna 52 at a predetermined pitch within a measurement plane P, and obtains far field directivity of the antenna 100 to be measured from distribution of amplitudes and phases within the measurement plane P. The display unit 57 displays the obtained directivity of the antenna 100 to be measured. Meanwhile, as the signal generator 54 and the amplitude and phase detector 55, a network analyzer having their functions can be used, and a personal computer can be used as the measurement control unit 56.
Here, in a case of the NFM, the probe antenna 52 scans the neighboring measurement plane P located away by approximately three wavelengths of a measurement signal from the antenna 100 to be measured, and thus the amplitude and phase of its electric field are detected.
The distribution of amplitudes and phases in the measurement plane P has a form of Fourier transformation of a function which is defined from the directivity of the antenna 100 to be measured and the directivity of the probe antenna 52. In the measurement control unit 56, the function is obtained by inverse Fourier transformation, and then arithmetic processing (probe correction) of removing the directivity of the probe antenna 52 is performed, thereby allowing the directivity of the antenna 100 to be measured to be obtained. In the measurement control unit 56, since data processing can be performed by fast Fourier transformation (FFT), it is possible to calculate the far field directivity of the antenna 100 to be measured at a fast rate.
As described above, it is generally known as disclosed in Non-Patent Document 1 that the distribution of amplitudes and phase in the measurement plane P has a form of Fourier transformation of a function which is defined from the directivity of the antenna to be measured and the directivity of the probe antenna, its function is obtained by inverse Fourier transformation, and then arithmetic processing (probe correction) of removing the directivity of the probe antenna is performed, thereby allowing the directivity of the antenna 100 to be measured to be obtained.
The NFM in which the directivity of the antenna is obtained in this manner has the following advantages over a far field measurement (FFM).
Since the NFM is a measurement in a short distance, a measurement can be performed even in a case where an anechoic chamber is not used, and a large-scale device is not required. In addition, since a device becomes compact in a millimeter-wave band, a measurement in a simple anechoic chamber installed in a living room can be performed, and it is possible to drastically shorten a time spent in constructing a measurement system which is a problem in a measurement in an anechoic chamber. Further, since a measurement in a region having a small free-space loss is performed, it is possible to obtain measurement results with good accuracy.
Further, in the NFM, the distribution of amplitudes and phases in the vicinity of an antenna is obtained. Therefore, in a case where the directivity as designed is not obtained, the cause can be diagnosed. This is a great advantage to a phased array antenna such as an active antenna.
However, in measurement devices of the related art that obtains a distribution of near field phases in the NFM, it is necessary to supply an antenna to be measured with a radio signal to radiate electromagnetic waves of the radio signal from the antenna to be measured, and to assign this radio signal, as a reference signal, to an amplitude and phase detector. On the other hand, a large number of active antennas have a problem in that, since an RF circuit and an antenna are formed integrally with each other, a terminal for inputting and outputting a signal to the antenna is not present, and that the reference signal is not able to be supplied from the RF circuit of the active antenna to the amplitude and phase detector.
A method as follows has been already proposed (for example, see Non-Patent Document 2). Probe antennas of which a plurality are two-dimensionally arranged scan the neighboring field of an antenna to be measured, and measure a phase difference between measurement positions adjacent to each other in a measurement plane. Thus, it is possible to measure the phase and the amplitude in the near field without supplying a reference signal from an antenna to be measured.
In the method disclosed in Non-Patent Document 2, a double-ridge waveguide tube having an opened one end is used as the probe antenna, in order to arrange probe antennas at a distance which is equal to or less than 0.5λ being a measurement distance required for a near field measurement. The double-ridge waveguide tube has an advantage in that electromagnetic waves of an equal frequency range are able to propagate with a cross-sectional shape smaller than the cross-sectional shape of a waveguide of a standard square waveguide tube.